;This is the code to get Poison Mushrooms functioning in SMB1. You will need to add these codes (denoted by my name, Shane) within the SMB1 disassembly by Doppleganger. Also, you need to assemble with X86 or ASM6.

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  .db $47, $47, $47, $47 ;brick (poison shroom) ;add this to Palette1_MTiles somewhere. What I did was remove an unused object in place to fit this.
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PowerUpObjHandler:
         ldx #$05                   ;set object offset for last slot in enemy object buffer
         stx ObjectOffset
         lda Enemy_State+5          ;check power-up object's state
         beq ExitPUp                ;if not set, branch to leave
         asl                        ;shift to check if d7 was set in object state
         bcc GrowThePowerUp         ;if not set, branch ahead to skip this part
         lda TimerControl           ;if master timer control set,
         bne RunPUSubs              ;branch ahead to enemy object routines
         lda PowerUpType            ;check power-up type
         beq ShroomM                ;if normal mushroom, branch ahead to move it
  ;;;;;SHANE       cmp #$03
  ;;;;;SHANE       beq ShroomM                ;if 1-up mushroom, branch ahead to move it
  ;;;;;SHANE       cmp #$04
  ;;;;;SHANE       beq ShroomM                ;if poison mushroom, branch ahead to move it
         cmp #$02
         bne RunPUSubs              ;if not star, branch elsewhere to skip movement
         jsr MoveJumpingEnemy       ;otherwise impose gravity on star power-up and make it jump
         jsr EnemyJump              ;note that green paratroopa shares the same code here 
         jmp RunPUSubs              ;then jump to other power-up subroutines


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.dw PoisonMushBlock ;add this to the routine labeled "BlockCode" at the end of it

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PoisonMushBlock:  ;;;;;add this whole labeled code somewhere of your choice
      lda #$04                ;load poison mushroom type
      .db $2c


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HandlePowerUpCollision:
      jsr EraseEnemyObject    ;erase the power-up object
  ;;;;;;;SHANE    lda PowerUpType  ;SHANE
  ;;;;;;;SHANE    cmp #$04                ;check power-up type 
  ;;;;;;;SHANE    bne Safe                ;if not a poison shroom, branch 
  ;;;;;;;SHANE    jmp InjurePlayer        ;otherwise injure the player properly ;SHANE
Safe: lda #$06 ;this value was in the original, I just gave it a label, now.

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.db $76, $77, $78, $79 ;poison mushroom ;add this to PowerUpGfxTable

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PowerUpAttributes:
      .db $02, $01, $02, $01, $03   ;The $03 at the end is now going to be the palette set for the poison mushroom.

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InjurePlayer:
      lda InjuryTimer          ;check again to see if injured invincibility timer is
 ;;;;;;;SHANE     ora StarInvincibleTimer  ;add this to make you immune to poison mushroom during star invincibility
	  bne ExInjColRoutines     ;at zero, and branch to leave if so

SetBounce:
ldy #$fa                   ;set a regular bounce rate for all other enemies
lda Enemy_ID,x
cmp #RedParatroopa         ;set a higher bounce rate for red paratroopas
beq BnceH                  ;and green paratroopas that fly
cmp #GreenParatroopaFly
bne BnceL

BnceH: ldy #$f8                   ;set player's vertical speed for bounce
BnceL: sty Player_Y_Speed         ;and then leave!!!
           rts